Automatically add relevent defines for C++11 support

When using C+11, e.g. by passing -std=c0x to gcc, precompiler definitions are added (_GXX_EXPERIMENTAL_CXX0X_ for g+) which exposes the associated standard library functionality.
QtCreator is not aware of these defines, and thus new functionality is not available in auto-completion and is marked as an error. To work arround this, I have to add
DEFINES = _GXX_EXPERIMENTAL_CXX0X_ in my .pro file if I'm using C+11.

Maybe the mkspeck should be updated to add the defines when needed for each supported compiler.